The recycling of wastepaper involves separation of usable pulp fiber from the other components of the wastepaper, such as mineral fillers, printing inks, laser toner particles, adhesives, etc. This is done through a series of steps that may be carried out in any way that is suitable to the purpose of the deinking plant and its customers. The desired product of this deinking process is deinked pulp (DIP).
Regardless of the deinking process employed, a composite waste material, sometimes called sludge or, alternatively, deinking residue (DIR), is produced with the DIP, and this DIR must be disposed or re-used in some fashion. DIR is comprised of three primary components: 1) fibers and other organic species, 2) inorganic species including minerals that are present as filling or coating pigments in the wastepaper, and 3) water. It is the preferred practice of some deinking plants to burn the DIR, which has a useful calorific value, to produce energy and steam. Burning the DIR simultaneously reduces the mass of solid waste that must be disposed or reused and converts it entirely to a mixture of inorganic minerals in an essentially dry state.
Some methods of treating DIR are described U.S. Pat. Nos. 5,759,258, 5,846,378, 6,063,237, and 6,830,615.
However, a need exists for a process to produce pigments for paper which utilizes recovered DIR and which requires limited milling or no milling.